In recent years light sources with broad spectra have immersed which appear to have a promising array of commercial application particularly due to relative high power with spatial properties suitable for coupling into fibers, such as single mode fibers. One class of such sources is referred to as supercontinuum light sources. Such sources are often designed by pumping a waveguide, often a fiber, with pulsed light having relatively high peak power and short pulse duration. The high peak power induces non-linear optical effects providing a broadening of the spectrum so that it may span one or more octaves. In this context so-called microstructured optical fibers are of particular interest as non-linear waveguides, as such fibers allow for large non-linear coefficient and posses a greater freedom than standard fibers in design of e.g. dispersion profiles, which may heavily influence supercontinuum generation. Supercontinuum light sources are known in for example from patents such as WO 2005/062113 and chapters 12 and 13 of R. Alfano: “The Supercontinuum Laser Source: Fundamentals with Updated References”, Springer, N.Y., USA, 2006.
Microstructured fibers are sometimes also referred to as photonic crystal fibers (PCFs) or holey fibers (a special case of such fibers is sometimes referred to as photonic bandgab fiber (PBG)). This PCF comprises a cladding made of a transparent material in which an array of holes is embedded along the length of the fiber [J. C. Knight, et al., Opt. Lett. 21 (1996) p. 1547. Errata: Opt. Lett. 22 (1997) p. 484]. The holes are commonly arranged transversely in a periodic array and are filled with a material which has a lower refractive index than the rest of the cladding. The centre of the fiber commonly comprises a transparent region, which breaks the periodicity of the cladding and this region often functions as the core of the fiber. However, in principle this region need not be in the centre of the cross section. Commonly the cross section of the fiber comprises a core region comprising a core region material, surrounded by a cladding region comprising holes (optionally filled with air or a gas), solid or liquid micro-structural elements embedded in a cladding background material, both regions extending in a longitudinal direction of the optical fiber. Commonly the core will guide 80% or more of the light in the operating wavelengths of the fiber. Typically, both the core and the cladding are made from pure fused silica and the holes are filled with air. In a variation thereof the PCF comprises transversely arranged rods of another material instead of holes. Such fibers are e.g. disclosed in WO37974 which also discloses the PCFs with transversely arranged holes.
The PCF type is generally produced from rod shaped units which are stacked to form a preform, which thereafter is drawn in one ore more steps to form the final optical fiber. In 2D Photonic band gap structures in fiber form”, T. A. Birks et al. “Photonic Band Gap Materials, Kluwer, 1996 is disclosed a method of producing a preform from rods in the form of capillary tubes by stacking the tubes. A method of fabrication of PCFs is also described in chapter IV, pp. 115-130 of “Photonic Crystal Fibers”, Kluwer Academic Press, 2003, by Bjarklev, Broeng, and Bjarklev.
WO 3078338 discloses a method of producing a preform for a microstructured optical fiber wherein a plurality of elongate elements are placed parallel to each other in a vessel where after at least a portion of said vessel is filled with a silica-containing sol, which is dried and sintered.